This invention relates to the deposition of tungsten-silicide (WSi.sub.x) films.
Refractory metal silicide films, such as tungsten silicide, are used in the manufacture of semiconductor integrated circuits such as schottky barriers, ohmic contacts and gate metallizations. Traditionally, polysilicon layers were used for these semiconductor circuits. However, advances in integrated circuit technology led to a scaling down of device dimensions, and an increase in chip size and complexity. Higher complexity chips such as VLSI's (very large-scale integrated circuits), require closely spaced inter-connection lines with smaller cross-sectional areas than conventional integrated circuits. The small cross-sectional area of the interconnection lines results in generation of more resistive heat. Close spacing results in less heat dissipation. This combination of more resistive heat generation and less heat dissipation can cause high temperatures which can result in part failure. Also, the higher resistivity increases the RC time constant which affects the delay time of the circuit. Low delay times are desirable for high speed circuits.
To overcome this problem, refractory metal silicides films having lower resistivities than polysilicon films were developed for use in these improved integrated circuits to obtain lower delay times and so that less heat is generated within the circuit. For gate metallizations, a low resistivity tungsten silicide film is deposited on top of a layer of polycrystalline silicon (polysilicon), to form a layered structure called a "polycide" structure.
At first, tungsten silicide films were deposited by physical vapor deposition techniques such as sputtering and electron-beam evaporation. However, these techniques gave films with poor conformal coverage over the steps and trenches of the polysilicon layer and non-uniform stoichiometry. Alternative deposition techniques such as low pressure chemical vapor deposition (LPCVD) were developed, and provided metallic silicide films with superior conformal coverage and superior properties.
Initially LPCVD processes for the deposition of tungsten silicide (Wsi.sub.x) films were based on the reduction of tungsten hexa-fluoride (WF.sub.6) by monosilane (SiH.sub.4). However, WSi.sub.x films produced using monosilane contained high levels of fluorine (often greater than 10.sup.20 fluorine atoms/cc). The high levels of fluorine led to degradation of these films due to migration of the fluorine atoms at operating temperatures. In addition, these films suffered from lower step-coverage and post-annealing problems which can lead to cracking and delamination of the tungsten-silicide layer.
Problems created by the silane chemistry can be avoided by using dichlorosilane (DCS), SiH.sub.2 Cl.sub.2, instead of monosilane. Tungsten-silicide films produced from the reduction of WF.sub.6 by DCS exhibit lower fluorine content, improved step coverage and stronger adhesion.
However, in spite of these encouraging results, the DCS/WF.sub.6 process has not been extensively adopted by the semiconductor industry because there are additional problems with these technologies.
First, these processes can fail to produce WSi.sub.x films with a uniform tungsten to silicon ratio through the thickness of the film. In the strata deposited in the initial stages of deposition (which later becomes the "interfacial layer" between the WSi.sub.x film being deposited and the substrate), the films deposited from current methods generally exhibit a value of "x" that is below the optimal range of 2.0 to 2.8. This phenomena is especially true for WSi.sub.x layers deposited on polysilicon. Values of x smaller than 2 (x=2 corresponds to the stoichiometry of the stable tungsten silicide, WSi.sub.2) in this interfacial strata are undesirable. The formation of an interfacial tungsten-rich strata can result in delamination of the WSi.sub.x layer during annealing of the fully processed wafer in the final stages of processing. Voids created by the migration of Si ions from the Si rich polysilicon layer to the Si deficient WSi.sub.x layer can also lead to poorer performance.
It is difficult to non-destructively detect formation of the tungsten-rich layer during the initial steps in the process of fabricating the integrated circuit chip. It is only in the final stages of the manufacturing process, when the fully processed wafers are worth between $50,000 to $100,000 each, that the delamination is discovered, and the entire wafer must be scrapped. This expense limits the use of current DCS/WF.sub.6 processes on an industrial scale.
Another problem with conventional CVD processes is that it is difficult to deposit a tungsten silicide film having a uniform sheet resistance. Typically, as a silicon content of the film is increased, the tungsten silicide film has a higher sheet resistance, and a greater non-uniformity in sheet resistance. Conventional tungsten silicide films have sheet resistances from about 500 to about 1500 .mu..OMEGA.-cm, and more typically from about 600 to about 1200 .mu..OMEGA.-cm. As the composition of the film is tailored to produce the higher sheet resistance, namely above 1000 .mu..OMEGA.-cm, the non-uniformity of the sheet resistance of the film also increases. Conventional CVD methods typically deposit WSi.sub.x films having sheet resistances with non-uniformities of about 1.5 to about 2.5%.
Another problem associated with use of current DCS/WF.sub.6 processes arises from the nucleation of silicon containing particles in the dichlorosilane gas phase. These particles accumulate and deposit on the walls of the deposition chamber. These deposits eventually fragment and generate particles which contaminate the wafer.
LPCVD equipment is typically sophisticated and expensive. Thus, new processes requiring substantial modification to existing equipment are avoided due to the high costs associated with such modifications. Accordingly, processes that can be implemented on conventional CVD equipment are highly desirable.
Therefore, there is a need for WSi.sub.x films having substantially uniform tungsten to silicon ratios through the thickness of the film and having low concentrations of fluorine. There is also a need for methods to deposit such films, and it is also desirable to be able to use conventional CVD equipment to deposit such films. There is also a need for a CVD process capable of depositing WSi.sub.x films having uniform sheet resistance, particularly at higher sheet resistivities.